nZVI-ryegrass integrated system for risk mitigation of chromium-PFAS co-contaminated soil: Pollutant immobilization and microbial niche regulation.

To address the high mobility, environmental risks, and challenges in synergistically managing co-contamination of chromium (Cr) and per-/polyfluoroalkyl substances (PFAS) in electroplating site soils, a cooperative strategy using nanoscale zero-valent iron (nZVI) and ryegrass was proposed and evaluated under laboratory-scale conditions. Combined application of 100 μg/g nZVI and ryegrass simultaneously suppressed the migration of both pollutants, reducing PFAS and total Cr leaching by 75.8% and 91.2%, respectively. The acid-extractable Cr fraction decreased from ∼35-4%, while the stable Cr fraction increased to 76%. Mechanistically, nZVI reduced Cr(VI) to Cr(III), promoted Cr root surface accumulation, alleviated Cr toxicity, and supplied iron to promote ryegrass growth, thereby enhancing plant uptake of PFAS from pore water and contributing to the plant-assisted stabilization of Cr. Transcriptomic analysis revealed that nZVI up-regulated key pathways related to cell membrane structure, energy-dependent transmembrane transport, ion homeostasis, cell wall composition, and oxidative stress response, collectively enhancing PFAS phytoaccumulation, resulting in 103%-145% increases in per-plant uptake of PFOS and 6:2 Cl-PFAES. Furthermore, the combined treatment drove a positive shift in the soil microbial niche, leading to more balanced Cr-reduction-associated dominant taxa and enrichment of nutrient transformation functional groups, which fostered a more stable and functionally directed soil microbial ecosystem. This study provides a theoretical and technical basis for effectively immobilizing Cr and PFAS mobility and steering a positive ecological trajectory in contaminated electroplating site soils.